Mobile terminals are widely used for wireless mobile communications of voice and/or data. As used herein, the term “mobile terminal” encompasses a wide variety of portable wireless devices that can access a cellular system. Mobile terminals include a cellular radiotelephone with a multi-line display, a Personal Communications System (PCS) terminal that may combine a cellular radiotelephone with data processing, facsimile and/or data communications capabilities, a Personal Digital Assistant (PDA) that can include a radiotelephone, pager, Internet/intranet access, Web browser, organizer, calendar and/or a Global Positioning System (GPS) receiver, and conventional laptop, palmtop and/or pervasive computing devices that include wireless receivers.
As is well known to those having skill in the art, in a terrestrial or satellite cellular system, one or more mobile terminals communicate with a plurality of cells that are served by base stations. A typical cellular system may include hundreds of cells, and may serve thousands of mobile terminals. The cells generally serve as nodes in the system from which links are established between mobile terminals and a Mobile Telephone Switching Office (MTSO) by way of the base stations serving the cells. Each cell may have allocated to it one or more dedicated control channels and one or more traffic channels. A control channel is a dedicated channel used for transmitting cell identification and paging information. The traffic channels carry the voice and/or data information. Through the cellular network, a duplexed radio communication link may be effected between two mobile terminals or between a mobile terminal and a land line terminal, through a Public Switched Telephone Network (PSTN).
Several types of access techniques are conventionally used to provide wireless services to users of cellular systems. Traditional analog cellular systems generally employ a system referred to as Frequency Division Multiple Access (FDMA), to create communications channels, wherein discrete frequency bands serve as channels over which mobile terminals communicate with base stations. Typically, these bands are reused in geographically separated cells in order to increase system capacity.
Modern digital cellular systems typically utilize different multiple access techniques, such as Time Division Multiple Access (TDMA) and/or Code Division Multiple Access (CDMA), to provide increased spectral efficiency. In TDMA systems, such as those conforming to the GSM or IS-136 Standards, carriers are divided into sequential time slots that are assigned to multiple channels, such that a plurality of channels may be multiplexed on a single carrier. CDMA systems, such as those conforming to the IS-95 Standard, achieve increased channel capacity by using “spread spectrum” techniques, wherein a channel is defined by modulating a data-modulated carrier signal by a unique spreading code, i.e., a code that spreads an original data-modulated carrier over a wide portion of the frequency spectrum in which the communications system operates.
Conventional spread spectrum CDMA communications systems commonly use so-called “Direct Sequence” (DS) spread spectrum modulation. In DS modulation, a data-modulated carrier is directly modulated by a spreading code or sequence before being amplified by a power amplifier and transmitted. However, other forms of spread spectrum modulation may be used.
When a mobile terminal in a cellular system is turned on, it generally searches for possible base stations with which to synchronize. In cellular systems, there are a number of possible radio channels or frequencies the base stations can use, and the mobile terminal may have to scan them all in order to find the best base station to use, in terms of signal strength and/or capacity. For example, in Wideband CDMA (WCDMA), there are about 300 possible radio channels separated by about 200 kHz, at about 1.9 GHz (uplink) and about 2.1 GHz (downlink).
Further, in WCDMA, some control channels transmitted from the base stations, called Primary Synchronization CHannel (P-SCH), Secondary Synchronization CHannel (S-SCH), and Common PIlot CHannel (CPICH), are used by the mobile terminal to find and detect a cell. In general, the initial cell search procedure may work as follows:
1. P-SCH is used in order to detect a new cell.
2. If a new cell is detected, S-SCH is used to find the timing and scrambling code for the new cell.
3. When the timing for the new cell is found, CPICH is used to measure the signal strength.
For more information about basic techniques for making cell search in WCDMA, see, for example, Wang et al., Cell Search in WCDMA, IEEE Journal on Selected Areas in Communications, Vol. 18, No. 8, 2000, pp. 1470–1482, the disclosure of which is incorporated by reference herein in its entirety as if set forth fully herein.
In principle, the mobile terminal may need to perform the cell search on each radio channel in order to be certain all base stations have been found. From these base stations, the best base station to use may be found.
When performing the initial cell search in systems such as WCDMA, it may take a long time to synchronize with the base station. In particular, it may take a long time to scan and perform a cell search on all radio channels, since the P-SCH and S-SCH channels generally are weak and detection statistics generally are quite low. Therefore, it may be desirable to shorten this search time.
Systems and methods for accelerated scanning of cellular channels are described in U.S. Pat. No. 6,205,334 B1 to Dent, entitled Accelerated Scanning of Cellular Channels by Cellular Radiotelephones. As described in the Abstract thereof, multiple-mode cellular radiotelephones use a wide bandwidth receiving mode while scanning for signals in a narrow bandwidth receiving mode. Thus, when it is desired to scan the received frequency band to search for the presence of narrowband signals, the wider receiver bandwidth is first selected. When significant signal energy is identified in the wider bandwidth, a further scan using the narrowband mode may then be provided in order to locate the narrow bandwidth channel containing the strongest signal. In another embodiment, the signals that are received in the wider bandwidth mode are digitized to obtain complex signal samples. The complex signal samples are then processed to determine energy in each of a plurality of narrower bandwidths corresponding to channels in the narrower bandwidth cellular radiotelephone standards. Accelerated scanning of TDMA cellular channels may be obtained by tuning the cellular radiotelephone to a succession of frequency channels within one TDMA time slot and measuring signal strength for each of the succession of frequency channels. Tuning and measuring is then repeated for remaining ones of the TDMA time slots in the TDMA frame, preferably using the same channels in the same order. For each frequency channel, the signal strength that is the greatest measured signal strength of the frequency channel in all of the TDMA slots is assigned to that frequency channel. The assigned signal strengths may then be used to select a frequency channel for TDMA signal acquisition. Historical information may also be used to accelerate scanning of cellular channels by a cellular radiotelephone.